The supply of natural gas liquid (NGL) in North America has become abundant because of the shale gas boom. This provides an opportunity to use NGL as a low cost feedstock for the production of transportation fuels and chemicals. Greater supply of shale oil also poses a challenge in meeting gasoline octane requirements, since shale oil-sourced naphthas inherently have low octane values. Efficient conversion of NGL to high octane gasoline and/or high cetane diesel fuel can help alleviate these problems.
At present, commercially-proven processes for upgrading light paraffins are centered around dehydrogenation. For example, the C3 and C4 Oleflex™ processes, produce propylene and iso-butene by dehydrogenation of propane and iso-butane feedstock, respectively, in a series of radial flow reactors. In addition, the Cyclar™ process converts liquefied petroleum gas (LPG) directly into liquid aromatics by dehydrocyclodimerization, which involves the sequential dehydrogenation of C3 and/or C4 alkanes to olefins, oligomerization of the olefins, cyclization to naphthenes and dehydrogenation of naphthenes to corresponding aromatics.
However, these processes have so far only been used for generating higher value chemical feedstocks because of the high capital and operating costs involved. In addition, they do not address the oversupply of ethane. There is therefore a need to develop a cost effective process for converting ethane in mixed light paraffin (C5-) streams to liquid fuels.
An alternative process for converting alkanes to alkenes is by selective oxidation, in which the alkane is catalytically dehydrogenated in the presence of oxygen. The process is also called oxidative dehydrogenation (ODH) and can be carried out at lower reaction temperatures than reductive dehydrogenation processes discussed above, and without the same problem of coke formation. For example, U.S. Pat. No. 8,519,210 discloses a process for the oxidative dehydrogenation of gaseous hydrocarbons, particularly ethane, to olefins, particularly ethylene. The process comprises contacting an ethane feed and an oxygen-containing gas in the presence of at least one of water and steam and an oxidative dehydrogenation catalyst comprising MoaVbNbcYdTeeOn wherein Y=Sb or Ni; a=1.0; b=0.05 to 1.0; c=0.001 to 1.0; d=0.001 to 1.0; e=0.001 to 0.5; and n is determined by the oxidation states of the other elements.
It is also known from, for example, U.S. Pat. Nos. 7,807,601 and 7,910,772, that light alkanes, especially propane can be selectively oxidized into unsaturated carboxylic acids, such as acrylic acid, in the presence of mixed-metal oxide catalysts having the formula MoaVbNbcTedSbeOf wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0, e=0.01 to 1.0, and f is dependent upon the oxidation state of the other elements.
A recent overview of the development of the selective oxidation of ethane and propane can be found in an article entitled “Oxidative dehydrogenation of ethane and propane: How far from commercial implementation?” by F. Cavani, N. Ballarini, and A. Cericola in Catalysis Today, vol. 127, Issues 1-4, 2007, pages 113-131.
However, although the selective oxidation of light alkanes has been extensively studied, the focus of the studies has been on the production of chemicals and chemical intermediates and, as reported in the Cavani et al. article, significant commercial utility has yet to be demonstrated.